In ophthalmic retinal surgery there exists a need for controlled vacuum. This vacuum can be used to remove debris, fluids or gases from the eye. It may also be used to manipulate membranes or other structures. When a vacuum is used to perform various tasks during surgery, different vacuum levels are required. Ideally, a surgeon is allowed to choose the appropriate vacuum level as the requirements change. This is normally accomplished through the use of a foot pedal. As the foot pedal is depressed, the vacuum level is increased proportionally. Most desirably, the vacuum level should track very closely to commands from the foot pedal or other control device.
Another requirement of the vacuum control system is to quickly and easily dispose of the aspirant (fluid or material suctioned or removed from the surgical process). Usually this is accomplished by flowing the aspirant into a bottle, bag or other container which generally has a volume of about 250 cc or more. The container is desirably transparent in order to aid in observation of the contents. These containers are commonly referred to as aspiration cassettes or aspiration bags. The aspiration cassette (cassette) should have enough capacity to capture the entire aspirant generated during a surgical procedure without overflowing or releasing fluids into the rest of the vacuum system.
Desirably, the cassette is simple to operate and inexpensive to manufacture as well as reliable during use. Since the vacuum control system is used for surgery, the system should operate quietly so as not to be a distraction.
In the past, various systems have been used to provide the vacuum control. One method uses a peristaltic pump. The aspiration line is connected to the surgical instrument at one end, runs through the pump and then empties into a bag or container at the other end. The vacuum is pulsed because of the roller action. It is difficult to measure and control the actual vacuum in the aspiration line without invading the line. The traditional method for changing the vacuum is to change the speed that the roller turns. It is difficult to control the pump speed rapidly enough to vary the vacuum synchronous with the changing requirements.
Most methods utilize a vacuum pump for a vacuum source. The vacuum is modulated by changing the pump speed or by using a variable orifice control for the vacuum. The vacuum is usually applied to a container to capture the aspirant. An aspiration line is connected to the container at one end and the surgical instrument at the other. In order for the vacuum to be responsive to the surgical requirements, the vacuum pump must evacuate the container volume. If the vacuum pump is sized so as to evacuate the volume quickly, the pump is prohibitively large. The most popular current method applies a Venturi as the vacuum source. This method requires a separate air source generally supplied by a tank of compressed nitrogen. In order to change the vacuum being generated, either the pressure to the Venturi can be modulated or the vacuum generated can be changed. As is the case with the vacuum pump, the vacuum is routed to a container. Since the system uses a separate pressure source, a change in vacuum is effected more quickly than with a vacuum pump, but the response time is still less than ideal. A higher flow Venturi could be used, but the increased input flow is objectionable. An internal air compressor could be included in the system, but like the vacuum pump, the compressor would be prohibitively large.
In order to increase vacuum responsiveness without increasing input air flow requirements, one system uses a cassette that is comprised of two containers. A smaller container has aspirant and controlled vacuum by a Venturi routed to it. When the system senses that the smaller container is almost full, it temporarily interrupts the vacuum. It then applies vacuum from a second Venturi to the larger container. This causes the aspirant to transfer from the smaller container into the larger one. Although this method works better than its forerunners, the vacuum response is still slower than desired, a separate pressure source is required, and the cassette mechanism is complex.
Objects to be met in the development of a new system would include: deposit of aspirant with no interruption of vacuum level; quick response of vacuum level when needed; sufficient containment space for aspirant; simple and inexpensive parts; and, requirement of only one vacuum source.